"clay" has a specific composition and contains something that is much like plastic, that gives it plasticity and helps to it hold together and then be fired into a vessle, unlike dirt that when heated withh crumble....i wrote a post on where i get mine and am kind of computer duh so hopefully this link works:
http://www.rawpaleoforum.com/health/eating-clay/msg24457/#msg24457also i would think if you would like to find some in your area find anyone interested in minerals/"rockhounding"
i was lucky enough to just be hanging out in the cave LOL and then decided the walls looked delicious and carved out chunks of the white clay with a knife and ate...i was with friends and we all emerged from the cave dirty with white shit all over our faces and lips, haha, crazed frothing dirt dwellers, but totally worth it!
as for the red clay for the skin i was hiking in this crazy rock formation park and it was after a few days of rain and i was playing in the run off and decided the red clay was beautiful and i needed to be that color, it stained my skin and was silty and smooth(gravity sifted?) and totally energizing to the point of insomnia